Step motors have been recently used in numbers of office automation apparatuses and computer peripherals such as printers, facsimile machines, image scanners, copying machines, laser beam printers and the like, as well as factory automation equipment including machine tools. The step motors are thus now extensively used both in applications and quantity, because an extremely simple and extraordinarily inexpensive system employing a step motor can perform a speed control or a positioning control.
FIG. 73A and FIG. 73B illustrate a construction and a circuit diagram of a conventional step-motor-driving device in accordance with a first conventional instance.
In FIG. 73A, two-phase step motor 801 includes two-phase driving windings, which are hereinafter referred to as phase-A winding and phase-B winding. Exciters 802a and 802b receive driving-instruction-signals DrA, DrB, thereby exciting phase-A winding and phase-B winding.
As shown in FIG. 73B, exciter 802a comprises a bridge circuit formed by four transistors, and this circuit works as follows: When input IN-A is at H level, output A takes H level and output /A takes L level. When input IN-A is at L level, output A takes L level and output /A takes H level. In other words, when input IN-A is at H level, a voltage VDC of dc current (not shown) allows the current to flow in plus direction, i.e. flowing from A to /A. When input IN-A is at L level, the voltage VDC allows the current to flow in minus direction, i.e. from /A to A. Another exciter 802b is identical to exciter 802a both in construction and operation.
On-delay circuits 811-814 (Dly) are provided as shown in FIG. 73B in order to prevent the transistor switches--making up the bridge circuit--from shorting the power supply by an error.
In FIGS. 73A and 73B, a plus voltage VDC is applied between inputs A and /A of exciter 802a when signal DrA is at H level. This voltage excites phase-A winding to be plus. On the other hand, a minus voltage VDC is applied between inputs A and /A when signal DrA is at L level, and this voltage excites phase-A winding to be minus. Phase-B winding excited by exciter 802b experiences the same phenomena as discussed above.
FIGS. 74A-74D illustrate an operation of the conventional motor driving device employed in the first conventional instance.
FIG. 74A illustrates relation between mover's position .theta. and torque T generated there, where torque T is obtained by providing the driving windings of motor 801 with a given excitation. The relations between position .theta. and torque T in respective instances are expressed in FIG. 74A as follows:
Ta: phase-A winding is excited to be plus; PA1 Th: phase-B winding is excited to be plus; PA1 -Ta: phase-A winding is excited to be minus; and PA1 -Tb: phase-B winding is excited to be minus. PA1 Ta+Th: phase-A and phase-B windings are excited to be plus; PA1 -Ta+Tb: phase-A winding is excited to be minus and phase-B winding is excited to be plus; PA1 -Ta-Tb: phase-A and phase-B windings are excited to be minus; and PA1 Ta-Tb: phase-A winding is excited to be plus and phase-B winding is excited to be minus. PA1 where, N=number of rotor teeth; PA1 .theta.ex=.theta.rot+.theta.lag+.pi./2N, should be established. PA1 .theta.ex=.theta.rot+.theta.lag-.pi./2N, should be established. PA1 (a) a step motor having driving windings for a plurality of phases; PA1 (b) exciters supplying power for exciting the driving windings; PA1 (c) position detectors which detect a position of a mover of the step motor, and then output a position detecting signal; PA1 (d) excitation-timing controllers which receive a driving instruction signal and the position detecting signal, and then output an exciting signal responsive to one of these two kinds of signals to the exciter. PA1 (a) a step motor having driving windings for a plurality of phases; PA1 (b) position detectors which detect a position of a mover of the step motor, and then output a position detecting signal; PA1 (c) exciters supplying power for exciting the driving windings; PA1 (d) an excitation current controller which supplies exciting current control signals as current value signals of the exciters, responsive to a phase difference between a driving instruction signal and the position detecting signal, to the exciter. PA1 (a) a step motor having driving windings for a plurality of phases; PA1 (b) exciters supplying power for exciting the driving windings; PA1 (c) position detectors which detect a position of a mover of the step motor, and then output a position detecting signal; PA1 (d) excitation-timing controllers which receive a driving instruction signal and the position detecting signal, and output an exciting signal responding to one of these two kinds of signals to the exciter. PA1 (d-1) a delay-signal selector which outputs a signal of later arrival out of the driving instruction signals and the position detecting signals when the step motor is driven in a positive direction, and which outputs a signal of later arrival out of the driving instruction signal and a reversed position detecting signal when the step-motor is driven in a negative direction; PA1 (d-2) a deviation detector which detects a difference between numbers of pulses, i.e. a number of pulses by the driving instruction signal and a number of pulses by the position detecting signal. In other words, this deviation detector sorts the motor condition into the following three statuses according to the difference in numbers of the pulses: A first status is that the deviation falls within a given range. A second status is that the deviation falls outside the given range and requires a forwardly-directed torque which induces the mover of the step-motor to move in the positive direction so that the deviation falls within the given range. A third status is that the deviation falls outside the given range and requires a reversely-directed torque which induces the mover to move in the negative direction so that the deviation falls within the given range; and PA1 (d-3) an exciting signal selector which selects--based on a detection result--an exciting signal out of an output signal supplied from the delay signal selector, the position detecting signal and the reversed position detecting signal. PA1 (1) selecting an output signal supplied from the delay signal selector as an exciting signal when the deviation detector detects the first status; PA1 (2) selecting the position detecting signal as the exciting signal when the deviation detector detects the second status; and PA1 (3) selecting the reversed position detecting signal as the exciting signal when the deviation detector detects the third status.
When the mover travels in positive direction, mover's position .theta. corresponds to the rightward direction in FIG. 74A, and when the mover travels in negative direction, it corresponds to the leftward direction in FIG. 74A. Regarding torque T, a forward direction allows the mover to travel in the positive direction and a reverse direction allows the mover to travel in the negative direction. The torque in the forward direction heads upper side of FIG. 74A.
In FIG. 74A, a torque generated by exciting the phase-A winding has a difference of 90-degree electrical angle from a torque generated by exciting the phase-B winding. Because driving windings of respective phases are mounted with a shift of 90-degree electrical angle with regard to polarity pitch of the mover.
FIG. 74B illustrates a relation between mover's position .theta. and torque T generated there by exciting both phase-A and phase-B windings (2-phase exciting drive). The relations between .theta. and T in respective instances in FIG. 74B are expressed as follows:
Each instance discussed above is a composite of each torque shown in FIG. 74A. Regarding mover's position .theta. and the direction of torque, FIG. 74B expresses them in the same manner as FIG. 74A does.
According to the above description regarding the step motor, it is roughly concluded that a relation between mover's position .theta. and torque T generated there can be uniquely determined when the driving windings of respective phases are excited to a given level.
FIG. 74C illustrates driving instruction signals DrA, DrB are fed, with 90 degree out of phase, into exciter 802a and 802b, and FIG. 74D illustrates how motor 801 works with these driving instructions.
Before time t1, since signals DrA, DrB are at H level, both phase-A and phase-B windings are excited to be plus, and torque "Ta+Th" drives the mover during this plus period. At time t1, when signal DrA changes from H level to L level, the polarity of exciting phase-A winding changes so that torque changes to be expressed "-Ta+Tb" and turns to the forward direction again. Due to these changes, the mover is further driven in the same (positive) direction until time t2 when signal DrB changes from H level to L level.
At time t2, when signal DrB changes from H level to L level, the polarity of exciting phase-B winding changes so that torque changes to be expressed "-Ta-Tb" and turns to the forward direction again. The mover is then further driven in the same (positive) direction without pause.
In the same manner, at time t3, t4 and t5, whenever signals DrA, DrB change, the polarities of exciting phase-A and phase-B windings alternately change so that torque changes to be expressed "Ta-Tb", "Ta+Tb", and "-Ta+Tb" and turns to the forward direction sequentially. The mover is thus kept driving in the same (positive) direction.
The first conventional step motor driving device operates as discussed above. In this first conventional instance, when the frequencies of signals DrA, DrB increase, the torque for driving the mover of the motor lowers so quick that the mover loses speed. As a result, the motor jumps out of synchronism.
There are two major factors for the motor to jump out of synchronism:
1. Inductance of driving windings affects the current to be insufficient for exciting the windings so that a desirable torque cannot be generated. Several methods have been proposed to overcome this factor. For example, a method of putting a series resistor in order to reduce electrical time constant of the windings, and a method of boosting a driving voltage temporarily to establish an exciting current quickly, were proposed. Another method is disclosed in the Japanese Patent Application Examined Publication No. S41-9489, i.e. a driving voltage is changed responsive to a frequency of a driving instruction signal.
2. A switch timing of exciting the windings is off the relation between the mover's position .theta. determined by excitation of the windings and torque T generated there. This second factor is described hereinafter with reference to FIGS. 75A-75D.
FIGS. 75A and 75B illustrate relations between the mover's position .theta. and torque T generated there when the windings of respective phases are excited. Those FIGS. are identical to FIGS. 74A and 74B.
Assume that driving instruction signals DrA, DrB, of which frequencies are higher than those shown in FIG. 74C, are fed to be exciting signals IN-A and IN-B into exciters as shown in FIG. 75C.
Signals DrA, DrB switch the direction of respective excitations applied to the driving windings, thereby generating a torque as illustrated in FIG. 75D.
Before time tr1, the excitation-change turns torque T to forward direction; however, when the frequencies of signals DrA, DrB are at high, torque T is eventually turned to reverse direction as happened at time tr1. This is because the excitation has been changed at a timing regardless of the relation between position .theta. and torque T generated there.
After time tr1, if the frequencies of signals DrA, DrB stay at high, torque T further turns to the reverse direction, and the mover consequently loses the driving torque and loses speed, then the motor jumps out of synchronism, and finally the motor stops.
When these two factors for "out of "synchronism" are compared, the "out of "synchronism" due to the second factor, i.e. timing of excitation-change to the windings, is often observed before the "out of synchronism" due to the first factor, i.e. inductance of windings. In other words, the second factor is the principal factor of the "out of synchronism".
FIG. 76 is a circuit diagram of a conventional step motor driving device in accordance with a second prior art.
In FIG. 76, two-phase step motor 801 includes two-phase driving windings, i.e. phase-A winding and phase-B winding. Exciters 802a and 802b receive driving-instruction-signals DrA, DrB, thereby exciting phase-A winding and phase-B winding.
Exciters 802a and 802b receive a constant voltage Vr as current-value-signals Ref-A and Ref-B. These signals control the maximum exciting current of respective phase-windings.
Exciters 802a comprises, as shown in FIG. 77, a bridge circuit formed by four transistors and current controller 815 (CL), and this bridge circuit works as follows: When input IN-A is at H level, output A takes H level and output /A takes L level. When input IN-A is at L level, output A takes L level and output /A takes H level. In other words, when input IN-A is at H level, a voltage VDC of dc current (not shown) allows the current to flow in plus direction, i.e. flowing from A to /A. When input IN-A is at L level, the voltage VDC allows the current to flow in minus direction, i.e. from /A to A.
Current controller 815 controls the maximum exciting current of phase-A winding of motor 801 so that the current can be determined by current value signal Ref-A.
In other words, when an exciting current of phase-A winding, which is detected by current-detection-resistor 816 (R), reaches the value indicated by current value signal Ref-A, RS flip-flop 818 is set, thereby changing outputs A and /A to H level. At this moment, the voltage applied to phase-A winding becomes substantially zero so that an exciting current can be kept at a constant value corresponding to Ref-A by an effect of the winding inductance. Oscillator 819 (OSC) resets RS flip-flop 818 periodically thereby applying a voltage to the winding. Oscillator 819 thus prevents the exciting current from attenuating due to inner loss and the like. Another exciter 802b operates in the same way as exciter 802a.
On-delay circuits 811-814 (Dly) are provided in FIG. 77 in order to w prevent the transistor switches--making up the bridge circuit--from shorting the power supply by an error. Filter 817 (Fil) is provided in order to remove noises appearing when the exciting current of the driving windings is detected by current detecting resistor 816 (R).
Plus voltage VDC is thus applied between inputs A and /A of exciter 802a when driving instruction signal DrA is at H level. This voltage excites phase-A winding to be plus. On the other hand, a minus voltage VDC is applied between inputs A and /A when signal DrA is at L level, and this voltage excites phase-A winding to be minus. In both the cases, the maximum exciting current of the windings is regulated with the value determined by current-value-signal Ref-A, i.e. Vr. Phase-B winding excited by exciter 802b experiences the same phenomena as discussed above.
FIGS. 78A-78D illustrate an operation of the conventional motor driving device employed in the second conventional instance.
FIG. 78A illustrates relation between mover's position .theta. and torque T generated there, where torque T is obtained by providing the driving windings of motor 801 with a given excitation. This is identical to the first prior art shown in FIG. 74A.
FIG. 78B illustrates a relation between mover's position .theta. and torque T generated there by exciting both phase-A and phase-B windings (2-phase exciting drive). This is identical to the first prior art shown in FIG. 74B.
FIG. 78C illustrates driving instruction signals DrA, DrB are fed, with 90 degree out of phase, into exciter 802a and 802b, and FIG. 78D illustrates how motor 801 works with these driving instructions.
Before time t1, since signals DrA, DrB are at H level, both phase-A and phase-B windings are excited to be plus, and torque "-Ta+Tb" drives the mover during this plus period. At time t1, when signal DrA changes from H level to L level, the polarity of exciting phase-A winding changes so that torque changes to be expressed "-Ta+Th" and turns to the forward direction. Due to these changes, the mover is further driven in the same (positive) direction until time t2 when signal DrB changes from H level to L level.
At time t2, when signal DrB changes from H level to L level, the polarity of exciting phase-B winding changes so that torque changes to be expressed "-Ta-Th" and turns to the forward direction again. The mover is then further driven in the same (positive) direction.
In the same manner, at time t3, t4 and t5, every time signals DrA, DrB change, the polarities of exciting phase-A and phase-B windings alternately change so that torque changes to be expressed "Ta-Tb", "Ta+Tb", and "-Ta+Tb" and turns to the forward direction sequentially. The mover is thus kept driving in the same (positive) direction without pause. The conventional step motor driving device in accordance with the second conventional instance operates as discussed above.
In this second conventional instance, ripples generated when the mover is driven are tremendously large as shown in FIG. 78D, and the large ripples boost vibrations, noises, and variations of rotational speed. This is because the mover is driven by the exciting current at a constant level.
In other words, since the "out of synchronism" is one of inherence of the step motor, torque margin should be provided enough not to invite the "out of synchronism" for driving the motor. For this purpose, an exciting current or a driving voltage of windings should be set, in general, at a greater value to a certain extent, namely, at a level where the exciting current of the windings can obtain a starting torque. In this case, a normal operation carries too heavy exciting current because the normal operation carries a smaller load than the starting time. As a result, as shown in FIG. 78D, ripples of the torque generated is obliged to be greater than the torque ripple at the normal operation.
Further, the heavy exciting current produces greater heat in the motor and lowers the efficiency.
When the frequencies of driving instruction signals DrA, DrB of the step motor driving device of the second conventional instance increases in the same way as the first conventional instance, the torque for driving the mover decreases rapidly, whereby the mover loses speed and then jumps out of sync.
A factor of this phenomenon is the same as that of the first conventional instance.
A switch timing of exciting the windings is off the relation between the mover's position .theta. determined by excitation of the windings and torque T generated there. The "out of sync." due to this factor is described hereinafter with reference to FIGS. 79A-79D.
FIGS. 79A and 79B illustrate a relation between the mover's position .theta. and torque T generated there when the windings of respective phases are excited. Those FIGS. are identical to FIGS. 78A and 78B.
Assume that driving instruction signals DrA, DrB, of which frequencies are higher than those shown in FIG. 78C, are fed into exciters as exciting signals IN-A and IN-B as shown in FIG. 79C.
Signals DrA, DrB switch the respective excitations applied to the driving windings, thereby generating a torque as illustrated in FIG. 79D.
Before time tr1, the excitation-change turns torque T to forward direction; however, when the frequencies of signals DrA, DrB are at high, torque T is eventually turned to reverse direction as happened at time tr1. This is because the excitation has been changed at a timing regardless of the relation between position .theta. and torque T generated there.
After time tr1, if the frequencies of signals DrA, DrB stay at high, torque T further turns to the reverse direction, and the mover consequently loses the driving torque and loses speed, then the motor jumps out of synchronism, and finally the motor stops.
The Japanese Patent Application Non-Examined Publication H07-59395 discloses the following driving method that intends to avoid the "out of sync" discussed in the first and second conventional instances.
In a step motor, of which rotor is driven by switching a phase to be excited following a driving instruction signal, when the relation of -.pi./2N&lt;(.theta.cmd-.theta.lag-.theta.rot)&lt;.pi./2N, is satisfied, .theta.ex=.theta.cmd should be established,
.theta.cmd=instructed position determined by an instruction pulse; PA2 .theta.rot=position of the rotor; PA2 .theta.ex=instructed excitation position; and PA2 .theta.lag=a delay angle between .theta.ex (instructed excitation position) and .theta.m (actual excited position).
When the relation of .pi./2N&lt;(.theta.cmd-.theta.lag-.theta.rot) is satisfied,
When the relation of -.pi./2N&gt;(.theta.cmd-.theta.lag-.theta.rot) is satisfied,
FIG. 80 is a block diagram illustrating a third conventional instance of a driving circuit of a step motor.
This prior art allows the step motor to work as a normal step motor when the motor is not subject to jumping "out of sync". However, the driving circuit induces the motor to function as a dc motor once the motor encounters "out of sync" so that the motor can pursue an instructed position with its maximum torque. After the motor catches the instructed position, the driving circuit restores the motor to the normal operation. As discussed above, this prior art discloses a method for preventing the motor from jumping "out of sync".
This prior art, however, requires rotor-position-detecting signals supplied from rotary encoder 902 having a high resolution in order to actualize itself. Because the rotor position ".theta.rot" should be finely detected in the range from -.pi./2N to .pi./2N, namely the threshold values of the respective formulae, (i.e. between -90 and +90 degree of electrical angle) for determining instructed-excitation-position ".theta.ex" by the formulae discussed above corresponding to respective cases. Within this range, if the rotor position is detected with a poor resolution of 1-2 pulses, due to this rough accuracy of position detection, although this rotor position allows the motor to rotate free from jumping "out of sync" even if the equation of .theta.ex=.theta.cmd is satisfied, the motor is actually driven by the maximum torque as if it were a dc motor based on an assumption of .theta.ex=.theta.rot+.theta.lag+.pi./2N being satisfied. On the contrary, although a rotor position needs to satisfy the equation of .theta.ex=.theta.rot+.theta.lag+.pi./2N in order to avoid jumping "out of sync", the step motor is driven based on .theta.ex=.theta.cmd thereby lowering the torque substantially. As such, the motor sometimes performs in a contradictory manner to what is disclosed by this prior art.
The Japanese Patent Application Non-examined Publication No. H011-262297 discloses an art embodying a controller of a step-motor based on this prior art. This has been published after the present invention (the part claiming a priority) was filed with the Japanese Patent Office. As this publication discloses, an encoder used as a position detector needs a resolution of 1,000 pulses per rotation (it is converted to 10 pulses within the range of between -90 and +90 degree of electrical angle).
In other words, the conventional step-motor-driving device including this prior art discussed above essentially needs a rotary encoder having a high resolution, such as a resolver, which makes the device bulky and expensive.